1. Field of the Invention
The present invention relates to fluid valve assemblies, and more particularly to a flow control valve assembly for a flow controller which can measure and control the mass flow of gases.
2. Description of the Related Art
The measurement and control of the mass flow of gases is important in many industries. During the manufacture of semiconductor chips, for example, many of the processes require precise reaction of two or more gases under carefully controlled conditions. Since chemical reactions occur at the molecular level, control of mass flow is the most direct way to regulate the absolute and relative quantities of gaseous reactants.
There have been developed in the art a variety of instruments for measuring the mass flow rate of gases from less than one standard cubic centimeter(s) per minute (seem) to more than 500,000 seem. The prevalent design of such instruments requires that flowing gas be routed through a sensor assembly, where the mass flow is measured, which includes a capillary tube around which are wound two resistance thermometers as identical as possible in electrical and mechanical characteristics. Each thermometer is wound in a tight coil in thermal communication with the outer surface of the tube. The thermometers form two legs of an electronic bridge; the other two legs are usually fixed resistors. When a voltage is applied across the bridge, current flows through each thermometer, causing it to self-heat. When there is no flow of gas through the capillary tube, the thermometers heat up identically. As gas begins to flow through the tube, the gas first cools the upstream thermometer and then the downstream thermometer, which is cooled less because the gas is now slightly warmer due to heating by the upstream thermometer. The resultant temperature differential is a function of both the mass flow rate and the properties of the particular gas.
After passing through the sensor assembly, the gas flows through a valve assembly which precisely controls the mass flow of the gas. In existing valve assemblies, the valve element typically may be either a ball that cooperates with a conical seat or a flat plate adapted to engage a raised and rounded scat. Balls are preferred because they are usually less expensive, simpler and more precise than flat plates. This is because there is a large supply of low cost standard balls made to a sphericity of 10 millionths of an inch or better on modern ball grinding machines. Also, it is very easy to produce ball seats to similar tolerances using simple coining techniques or commercially available jewels. Conversely, flat valve elements are specially made and do not benefit from standardization. The raised and rounded seats used for flat valves are difficult to make and even more difficult to repair if physically damaged.
In any case the valve element and seat must be sealed from impurities in the external atmosphere. This is sometimes accomplished by either a flexible diaphragm or bellows welded in place. Gas flow is controlled by an external actuator operable to press on the flexible member to seat the valve element and thereby close the valve. Differential gas pressure or a spring provides the opening force.
Some semiconductor manufacturing processes, such as ion implantation, require that precise small portions of gas be admitted into a vacuum chamber, so the mass flow controller must be able to operate at very low pressure. Other processes require gases at high inlet pressures. Thus, there is a need for a valve assembly which can operate reliably and precisely over a wide range of flow rates and system pressures.
In the manufacture of semiconductor devices, and especially those having features of one micron or less, the reactant gas must not only be carefully controlled but also completely free from contaminants. Particles such as dust, metal and lint, vapors from moisture, solvents and oil, and contaminant gases such as air and extraneous process gases can spoil the products. It is therefore important that the flow passages used in mass flow controllers neither trap such contaminants and subsequently release them to the gas stream, nor generate contaminants during calibration and operation.
Typically, friction causes gas valves to deteriorate and generate undesirable small particles which contaminate the reactant gases. One source of contamination in existing gas valves is the frictional engagement between the ball valve and the walls of the guiding members retaining the ball. Particles thus generated may become trapped in the pocket surrounding the ball and resist removal by purge gases periodically introduced to sweep the gas path. Also, when valves are used as part of a control system to regulate the flow of gases, friction and particulate matter can cause undesirable hysteresis in the control system.
U.S. Pat. No. 5,165,655 ("'655"), entitled "Flow Control Valve Assembly Minimizing Generation and Entrapment of Contaminants," which is incorporated herein by reference in its entirety, discloses four embodiments of a valve assembly including a valve element retainer permitting alignment between a ball valve element and its seat, while minimizing friction and the generation and entrapment of particles which could affect the chemical process or impede ball motion. In all the embodiments, the inlet port through which gas enters the valve assembly terminates in a valve seat, and the ball is either loosely coupled or rigidly attached to a first portion of a flexible retainer which guides the motion of the ball, relative to the seat, between its open and closed positions. A second portion, extending from the first, is rigidly attached to the valve assembly body.
Although the retainers in these embodiments can provide substantial radial restraint on a ball element to maintain alignment, it was found impractical to hold the manufacturing tolerances on elements close enough for the valve assemblies to work reliably. Another shortcoming of these embodiments is that with the ball in the normal (closed) position, contaminant deposits tend to build up on the ball lower surface and seat at the small space between the ball and seat so that the ball adheres to the seat. Particularly troublesome is tungsten hexafluoride (WF.sub.6) gas which reacts with minute traces of water vapor to form tungsten tetrafluoride (WF.sub.4), which solidifies to form a strong glue. It was found that if the retainer has a low spring rate, to allow self-centering, it is too weak to break the adhesive bond, necessitating a blast of high pressure gas to free the ball. If a spring to lift the ball is not provided, in some applications where the mass flow controller is interposed between an adsorption-type storage container of process gas and a vacuum chamber, gas typically will cease flowing when the inlet pressure falls below 2 Torr, because that much pressure is needed to lift the ball off the seat. Because the amount of gas in such containers varies non-linearly with pressure, it is essential to be able to operate at the lowest possible pressure to avoid wasting gas and having frequent shutdowns.
It is therefore an overall object of the present invention to provide a valve assembly for a mass flow controller which can, over a wide range of system pressures down to a hard vacuum, measure and control the mass flow rates of gases used in semiconductor fabrication processes.
A more specific object of the invention is to provide a valve assembly wherein a ball valve element can be precisely and reproducibly centered within a valve seat.
A further object of the invention is to provide a valve assembly in which hysteresis due to valve element friction is minimized.
Still another object of the invention is to provide a valve assembly for a mass flow controller which can admit into a high-vacuum chamber substantially all the adsorbed gas stored in a container.
Other objects of the invention will become evident when the following description is considered with the accompanying drawings.